mfd: rave-sp: Add code to print firmware versions

Add code that would query and print out bootloader and application
firmware version info.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Tested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>